1. Field of the Invention
The present invention relates to an image information reading method and an image information reading apparatus. More specifically, the present invention relates to a method and an apparatus for reading image information by using a solid-state image detector having a semiconductor in a main part thereof.
2. Description of the Related Art
In medical radiography aimed at medical diagnoses, radiation image information reading apparatus using radiography films or stimulable phosphor sheets have been known.
In medical radiography, various kinds of radiation image recording and reading apparatus using solid-state radiation detectors have also been proposed and put into practice. In a solid-state radiation detector, an electric charge obtained by detecting radiation is stored in a capacitor of a detection device and the electric charge is output by being converted into an electric signal representing radiation information. As a solid-state radiation detector used in such apparatus, various types have been proposed. With respect to an electric charge generating process in which radiation is converted into an electric charge, solid-state radiation detectors employing an optical conversion method and a direct conversion method are available. With respect to an electric charge reading process in which an electric charge stored in a detector is read, solid-state radiation detectors are classified into TFT (Thin Film Transistor) reading type detectors and optical reading type detectors.
An optical conversion type detector comprises a solid-state detecting unit (an image reading unit) having a plurality of photoelectric conversion devices formed on an insulating substrate and phosphor formed on the solid-state detecting unit. In a detector of this type, the photoelectric conversion devices detect light emitted from the phosphor by exposing the phosphor to radiation, and a signal electric charge thereby obtained is stored in capacitors of the detector. The stored electric charge is then converted into an electric signal and the signal is output (see Japanese Unexamined Patent Publication Nos. 59(1984)-211263 and 2(1990)-164067, PCT International Publication No. WO92/06501, and SPIE Vol. 1443 Medical Imaging V; Image Physics (1991), p. 108–119, for example).
A direct conversion type detector comprises a solid-state radiation detecting unit having a plurality of electric charge collecting electrodes formed on an insulating substrate and layers of radiation-conductive material formed on the charge collecting electrodes and generating an electric charge representing radiation information by being exposed to radiation. In a detector of direct conversion type, the signal electric charge generated within the radiation-conductive material by exposing the material to radiation is collected by the electric charge collecting electrodes and stored in capacitors. The stored electric charge is then converted into an electric signal, and the signal is output (see MATERIAL PARAMETERS IN THICK HYDROGENATED AMORPHOUS SILICON RADIATION DETECTORS, Lawrence Berkeley Laboratory. University of California, Berkeley, Calif. 94720 Xerox Parc. Palo Alto. Calif. 94304, Metal/Amorphous Silicon Multilayer Radiation Detectors, IEEE TRANSACTIONS ON NUCLEAR SCIENCE. VOL. 36. NO.2. APRIL 1989, and Japanese Unexamined Patent Publication No. 1(1989)-216290, for example). The electric charge collecting electrodes and the radiation-conductive material comprise a main part of solid-state detecting devices using the direct conversion method.
In the TFT reading method regarding the electric charge reading process, a signal electric charge stored in capacitors is read while switches such as TFTs set on a signal line connected to the capacitors of detecting devices are turned on in a scanning manner. The optical conversion type detectors and the direct conversion type detectors generally use this method. The switches can take other forms, although TFTs are generally used.
On the other hand, the optical reading method is a method of reading a signal electric charge stored in a capacitor by irradiating light for reading (an electromagnetic wave for reading) on a solid-state detecting device. A portion of direct conversion type detectors uses this reading method with respect to the charge generating process.
The applicant has proposed a solid-state radiation detector by improving a direct conversion type radiation detector (see Japanese Patent Application No. 9(1997)-222114).
The radiation detector of improved direct conversion type comprises a first conductive layer which is transparent to radiation for recording, a photoconductive layer for recording exhibiting conductivity when receiving the radiation for recording which has passed through the first conductive layer, an electric charge transport layer which acts substantially as an insulator to an electric charge having the same polarity as an electric charge charged in the first conductive layer while acting substantially as a conductor to an electric charge having the reverse polarity, a photoconductive layer for reading exhibiting conductivity when receiving an electromagnetic wave for reading, and a second conductive layer which is transparent to the electromagnetic wave for reading, with these layers being disposed in this order. A latent image electric charge representing image information is stored at the interface between the photoconductive layer for recording and the electric charge transport layer. This detector employs the direct conversion method with respect to the electric charge generating process and the optical reading method with respect to the electric charge reading process.
As methods of reading the latent image charge in a solid-state radiation detector using the improved direct conversion method, the following methods are known. For example, a flat shape is adopted for the second conductive layer (reading electrode), and the latent image electric charge is detected by scanning the reading electrode with spot-like reading light such as a laser beam. Alternatively, an electrode in a comb-like shape (stripe electrode) is used as the reading electrode and a linear light source elongated along the direction almost orthogonal to the longitudinal direction of the stripe electrode scans the stripe electrode longitudinally to detect the latent image charge.
However, since the radiation detectors of any methods described above have a limit in a maximum electric charge (saturation charge) that can be stored or read, an A/D conversion range for digitizing the image signal is set narrow. Therefore, a saturation level of the signal is lower than in an apparatus using a conventional radiography film or a stimulable phosphor sheet, and a dynamic range thereof is also small. When image data obtained by digitizing the image signal output from the solid-state detecting devices are visualized, density and contrast of the image do not necessarily become adequate. As a result, quality of a radiation image obtained in this manner is lower than a radiation image obtained by an apparatus using a conventional radiography film or the like.
For example, at the time of low radiation dose photographing, the signal range becomes small and the signal cannot be used to a full accuracy of A/D conversion. Therefore, bit resolution becomes lower and quantization noise becomes conspicuous. Furthermore, noise caused by an electric system such as a detector or a recording and reading apparatus becomes large, and an S/N ratio decreases.
Moreover, since the A/D conversion range cannot be set in accordance with the image signal range, a high-bit A/D converter is necessary for an adequate bit resolution in both low radiation dose photographing and high radiation dose photographing.